The fate and transport of uranium in subsurface environments is often controlled by uranium association with solid phases.  This association can occur through both adsorption of uranium to mineral surfaces to form surface complexes and through the precipitation of uranium-containing precipitates in solution or on substrate minerals.  With increasing uranium loadings, the mechanism of uptake can transition from adsorption to precipitation.  This transition is illustrated in experiments on (1) the influence of phosphate on the sorption of U(VI) to goethite and montmorillonite and (2) the adsorption of U(IV) to magnetite and rutile.  Laboratory experiments on the influence of phosphate on U(VI) sorption combined macroscopic sorption measurements, equilibrium speciation modeling, X-ray absorption spectroscopy, and laser induced fluorescence spectroscopy to identify the specific conditions at which U(VI) phosphate precipitates formed.  The study generated spectroscopic evidence for ternary U(VI)-phosphate-mineral surface complexes that were present prior to achieving sufficient supersaturation to nucleate U(VI) phosphate precipitates.  In environmental remediation efforts involving the reduction of U(VI) to U(IV), the low solubility of U(IV) makes UO<sub>2</sub> precipitates a likely end product.  However, recent work has identified a range of non-crystalline U(IV) species that can also form from U(VI) reduction.  A set of controlled laboratory experiments quantified U(IV) adsorption to magnetite and rutile. The experimental data were used to develop equilibrium models for prediction of U(IV) uptake with solid phases by both adsorption and precipitation.

Transition from adsorption to precipitation for uranium uptake on geomedia

251st National Meeting (2016)

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